Probably this bit from the Sanity sub-system -
Each time a character encounters a Great Old One
Save DC = 15 + CR of the creature
Failure: Sanity damage equal to double the creature’s CR
Success: Sanity damage equal to the creature’s CR

EDIT: For other GMs -- the reference above is to the Horror Adventures Horror Characters >> Sanity (page 13 in the hardcover). The reason I specify is there is ALSO a chapter on Sanity much later in the book, which was where I was trying to find the pertinent information.

Ah. Gotcha. Yeah, not using that subsystem. My players have already resorted to arson, eating rats, murdering Losandro with telekinetic projectiles of her own stuff, and serious discussions about cannibalism.